Question

什么情况下，不定式中的to可以省掉

Answer 1

1.当两个或两个以上的不定式并列在一起时,第一个不定式带to,后面的不定式往往省略to,参见下文首
例；但是,如果是在对照的场合,就不可以省去to,
参见下文后例.
He told me to finish my homework and（to） hand it in.
他对我说：“完成作业并把它交上去”.
It is better to laugh than to cry.
笑比哭好.
2.在感官动词see,watch,look at,hear,listen to,feel和使役动词make,have,let等所接的宾语中（不定式）作宾语补足语）,不定式应省去to.但是在被动语态中,则to不能省掉.
注意
疑问词+不定式作宾语和宾语补足语时,常与下列动词连用：know,show,teach,tell,find out,learn,decide,forget等.
句型转换
简单句：
I don't know what to do.
复合句：
I don't know what I should do.
我不知道要做什么.
how的用法
How old are you?
你多大了?
How are you?
How do you do?
How tall he is!
他多高啊!
（感叹句）
How did you come here?
你怎么来这儿的?
（对状语提问）
I saw him dance.
被动：He was seen to dance.
我看到他跳舞.
The boss made them work the whole night.
老板迫使他们工作了一整夜.
被动：They were made to work the whole night.
Let me listen to you sing the song.
让我听你唱这首歌.
被动：You will be listened to sing the song（by me）.
3.help后面的不定式可带to,也可不带to.
I often help him（to）clean the room.
我常常帮他打扫房间.

Answer 2

不定式省略to 的情况 不能统一的情况，只能够死记，一般来说 Let/ have /get 等高频单词要记住